I. Field of the Invention
This invention relates to improvements in solder mounting of electrical contacts and in a process enabling the mounting of electrical contacts in accordance with the invention.
II. Description of the Prior Art
It has become more and more commonplace for electronic components to be surface mounted on printed circuit boards (PCBs). This is for a number of reasons including the ability to achieve a higher density of components on a board and because it is easier for robots to be used and achieve the desired end results. At the same time, there remains a need for certain components still to be pin connected. This might occur, for example, when the density of pins does not allow for both pads and vias (that is, plated through holes used to contact circuits at various levels of a multi-level circuit board). Also, this might occur in multiple row connectors, as with a four row connector, in which it might be impossible to direct the middle two rows between the outer two rows so as to accomplish inspectable surface termination.
In such mixed technology PCB assemblies, however, there was a problem during the assembly operation since wave soldering is a preferred method of attaching pin mounts while reflow soldering is a preferred method for surface mounting. In a typical wave soldering operation, the underside of a printed circuit board to which a component has been attached with the pins extending through and beyond mounting holes provided therein, is advanced transversely toward a standing wave of molten solder. As the PCB advances, the solder wave envelopes the pins and, by reason of capillary action, the solder joins each pin with its associated metal plated hole in the PCB.
In a reflow operation which, as noted above, is generally utilized for surface mounting of components on a PCB, a layer of solder paste which is a slurry mixture of tin and lead powder, carrier material, and flux is then applied to the surface of the PCB. The contacts of a component to be mounted on the PBC are then pushed into the solder paste which is sufficiently thick and viscous to hold the component in place until a subsequent reflow operation is performed. In the reflow operation, the PCB, together with its component positioned thereon, is then heated to a temperature at which the solder in the solder paste melts, then resolidifies when heat is withdrawn from the assembly. When the assembly is cooled, the solder hardens with the result that the component is firmly mounted to the PCB in its desired location.
Because of the diversity of the two processes customarily used for mounting technology on a PCB, it was generally required, in those instances in which mixed technology was to be mounted on a PCB, to perform two separate steps in such a manner that the following step would not interfere with the results of the preceding step. It became customary, therefore, to first perform wave soldering in order to mount the components having pin contacts, then to perform a reflow operation for applying the surface mounted components to the PCB. However, it will be appreciated that the two step process just described is time consuming. Furthermore, even though precautions are taken, it is possible for the subsequent reflow operation to have a deleterious effect on the earlier solder wave operation. There is also the possibility of damage from heat or handling.
With the passage of time, therefore, it came to be recognized that it would be desirable to perform only one mounted operation for all components, whether they be of the surface mounted type or of the pin mounted type. In order to achieve this end, however, it was necessary to provide in each connector location a predetermined amount of solder and flux. This was achieved by means of solder pre-forms of various shapes and sizes which were applied to or placed adjacent the contacts preliminary to a reflow operation. For example, washer-shaped pre-forms were sometimes fashioned in successive layers, alternately, of flux and solder, then fittingly attached to a pin adjacent to a desired point of electrical connection. Sometimes a particular shape is due to application geometry, and sometimes merely a convenience for the pre-form vendor. Other typical pre-forms were square shaped and still others were resin core solder, that is, a long or continuous tube of solder surrounding a flux core. In this latter instance, short lengths of resin core solder would be cut off or formed and placed so as to be mechanically held by the connector pin of a component pending the reflow operation. An example of this is the Solder Bearing Lead (SBL) Series of connectors manufactured by Teka Interconnection Systems of Providence, R.I.
In another attempt to obtain a predetermined amount of solder on a contact tail, it was washed in a solution of a type repellent to solder. In this instance, the solder would only adhere to the non treated area. The contact tail was then passed through a solder bath or wave thereby in resulting in some measure of solder thereon, but the resulting globule of solder which adhered to the contact tail was not an accurately predetermined amount and would vary to a substantial extent from one contact tail to the next.
It was with knowledge of the current state of the art as represented by the technology just described that the present invention was conceived and has now been reduced to practice.